While drilling for underground hydrocarbon formations, many material properties may be logged to allow a driller to better understand the underground formation. Material properties may be determined utilizing electromagnetic waves emitted and received by a measurement device in a downhole tool. One such material property is resistivity. Resistivity measurement tools generally allow the driller to determine the resistivity or resistance to the conduction of electricity of the surrounding formation. By knowing the resistivity of the surrounding formation, an operator can make determinations about the makeup of the formation including, for example and without limitation, the presence or absence of water or hydrocarbons, as well as the porosity and/or permeability of the formation.
For some downhole tools which utilize electromagnetic waves, the measurement device is positioned within a tubular segment or collar referred to herein as a tool collar. The collar may be positioned as part of a tubular string including a plurality of tubular segments. By including the measurement device as part of the drilling string, measurements may be made during the drilling operation. In some tools, the measurement device is formed as a probe or sonde located within the collar. In other downhole tools, the measurement device may be coupled to but located within the collar. Unlike external antenna tools, in which the conductors are positioned on the exterior of the collar, a probe based tool or internally located tool may include antenna elements located internal to the collar.
Internal antenna tools may operate by transmitting an electromagnetic field through the formation between one or more transmitter antennae to one or more receiver antennae spaced apart along the tool string. One or more slots may be formed through the wall of the collar corresponding generally with the placement of each antenna of the probe. Because the collar is made of a conductive material, the electromagnetic field would be attenuated if not blocked altogether from entering the surrounding formation without the slots. Because of the necessity to maintain differential pressure between the interior and exterior of the tool string, the slots must be sealed with a resilient yet electromagnetically transparent plug. Probe based resistivity tools are discussed in more detail in U.S. Pat. No. 6,483,310, filed Oct. 17, 2000 (“Retrievable, formation resistivity tool, having a slotted collar”), the entirety of which is hereby incorporated by reference.